Thermal processing furnaces or reactors are commonly used for batch processing semiconductor wafers during several fabrication stages of silicon integrated circuits. Processing steps for which a furnace may be used include oxidation, diffusion, annealing and chemical vapor deposition (CVD) and pulsed atomic layer deposition (ALD).
A conventional vertical thermal processing furnace may include a thermally resistive heating coil, powered by an electrical power supply. Within the heating coil there may be provided a bell jar-shaped outer reaction tube and an inner reaction tube that is coaxially disposed within the outer reaction tube. The inner reaction tube is commonly referred to as the liner. The lower end of the outer reaction tube may be open, while the upper end thereof may be closed, typically by a dome-shaped structure. The liner may be open at both its lower and upper end. The lower ends of both the outer reaction tube and the liner may be supported on a flange, which may define a central furnace opening via which a wafer boat holding a plurality of wafers may enter and exit the reaction chamber that is formed by the interior of the liner. The wafer boat may be supported on a thermally insulating pedestal, which in turn may be supported on a door plate that may serve to close off the central furnace opening in the flange. The flange may further be provided with a gas feed conduit that connects to a gas injector disposed inside the liner, and a gas exhaust conduit via which a vacuum pump may be connected to a lower end of the gas passage that exists between an outer wall of the liner and an inner wall of the outer reaction tube.
In operation, a wafer boat may be introduced into the reaction chamber, which may then be evacuated. Subsequently, a process gas may be fed to the reaction chamber via the gas feed conduit and the gas injector. The process gas may flow upwardly within the inner reaction tube while contacting the wafers provided therein. As the process gas exits the upper end of the inner reaction tube and reaches the closed upper end of the outer reaction tube, it may reverse its direction and flow downwardly through the gas passage between the inner and outer reaction tubes, so as to be exhausted from the reaction chamber via the gas exhaust conduit by the vacuum pump.
A general problem associated with thermal processing furnaces is contamination of the reaction chamber atmosphere with small particles. A particle that ends up on a wafer being processed may render a die to be manufactured therefrom inoperable. Contamination of the reaction chamber atmosphere may have different causes.
U.S. Pat. No. 7,736,437 expresses the belief that the dome of a quartz, bell jar-shaped outer reaction tube generates a significant number of micro-particles when it is heated. In operation, these particles may fall into the upper, open end of the inner reaction tube and thence onto the wafer boat and the wafers supported therein. To prevent this, U.S. '437 teaches the use of a cover that is to be disposed on top of the inner reaction tube. The cover may include apertures, such as convolute passageways, in order to allow a substantial upward flow of process gas therethrough while blocking most particles from the dome from falling to within the inner reaction tube.
U.S. Pat. No. 6,503,079 identifies the furnace opening portion of a thermal processing furnace, and more in particular outgassing O-rings, leaking seals and wafer boat rotation mechanisms in the furnace opening portion, as possible sources of contamination. Since the furnace opening portion is located upstream of a flow of process gas within the reaction chamber, the process gas introduced into the reaction chamber may serve as a carrier for the contaminants generated by the aforementioned sources in the furnace opening portion. The contaminants may thus be deposited on and/or adhere to any wafers present in the reaction chamber. Obviously, this may hinder film growth and inhibit process reactions, and so form a cause of poor film quality. To prevent contamination of the reaction chamber, U.S. '079 teaches the use of a reverse-diffusion preventing body, disposed between the reaction chamber and a furnace opening portion space at a side of the furnace opening, within the furnace. In addition, two independently operable gas exhaust systems are provided: a process gas exhaust system for exhausting process gas from the reaction chamber, and a purge gas exhaust system for exhausting purge gas from the furnace opening portion space. The contaminating furnace opening portion is thus being isolated from the reaction chamber by the reverse-diffusion preventing body, while both the reaction chamber and the furnace opening portion space are both provided with their own gas flow management. This combination of features makes it possible to prevent diffusion of a contaminant from the furnace opening portion space to the reaction chamber, and hence to the wafers.